This invention relates to a unique way of testing a switch to determine whether the switch will provide a desired feel to an operator.
Switches are utilized in many control functions. Various types of switches are moved by an operator between any one of several positions to terminate or begin operation of a system, component, etc. Switches are tested to insure that they do not present unduly high resistance to an operator. That is, it is not desirable to have a switch that is difficult to move.
FIG. 1 graphically illustrates the typical testing that has been performed on a switch design. The force resistance of the switch is plotted with respect to the movement of the switch. Typically, a switch has greater forces as it approaches an end of travel or detent position. Historically, switch designers have looked only to the magnitude of the force. As an example, FIG. 1 shows an example of two switch tests which plot the resistance force against movement of the switch. A graph 20 includes acceptable envelope boundaries 22 and 24 which are plotted onto the force versus movement graph 20. In the prior art, a switch design is found unacceptable if the force should cross the boundaries. Thus, a first switch design with test results 26 would be found acceptable since the plot is within the boundaries 22 and 24 throughout its range. Note that the graph 26 has extreme low points 28 and high points 30, and fluctuates repeatedly between those points.
In fact, while this switch design would be found acceptable, the feel might well be undesirable to an operator. The rapidly fluctuating force would make it difficult for an operator to determine end of travel, or whether the switch has been moved sufficiently to a particular position. Moreover, such rapidly fluctuating resistance force is typically not found to provide a good feel to the operator.
A second plot 32 is also shown in the graph 20. Plot 32 represents a second switch test, and does not have the rapid fluctuations of the plot 26. However, there is an extreme high point 34 in plot 32. In fact, plot 32 moves gradually upwardly to the high point 34 and then decreases gradually again. Using the prior art switch testing methods, the plot 32 would be found to indicate the associated switch was unacceptable. The high point 34 is outside of the boundary 24, and thus this switch would be rejected or reworked.
In fact, most operators might well find the switch shown by the plot 32 to feel better than the switch shown by plot 26. Rapid fluctuations, outside detent or end of travel positions, are much less desirable than a gradual change. Thus, the prior art type testing illustrated in FIG. 1 does not provide fully accurate information of a switch feel.
One prior art attempt to address this problem is illustrated in FIG. 2. FIG. 2 shows a second graph 36 having force boundaries 38 and 40 which are much closer than those shown in FIG. 1. A plot 42 for a switch must fall within the boundary 38 or 40 or the switch will be found unacceptable. By making the boundaries 38 and 40 quite close, the switch designers hope to minimize fluctuation. Even so, some fluctuation still exists. Moreover, by making such tight boundaries, otherwise acceptable feeling switches are labeled unacceptable.
In a disclosed embodiment of this invention, a method of testing a switch focuses on the xe2x80x9cfeelxe2x80x9d to the operator by looking at how the resistance force changes with movement. The present invention has determined that the most relevant factor to an operator""s feel is whether the change in resistance force is gradual, like plot 32, or extreme, like plot 26. Thus, the present invention plots the resistance force against movement of the switch, and then looks at the second derivative of that plot. It is desirable to keep the second derivative as close to zero as possible, except at detents or end of travel positions to provide a smooth, well-defined feel.
In the disclosed embodiment of this invention, the present invention uses an upper and lower acceptable limit to the second derivative plot. If that second derivative plot crosses one of the limits, then the switch is found unacceptable in the region where the second derivative has crossed the limits. It is typical that the second derivative will have spikes at detents or end of travel position. According to the present invention, a second derivative spike wherein the second derivative plot moves far from zero at a location other than the end of travel or detent that could provide an undesirable feel. If the problem occurs with a design being tested, a designer may wish to reevaluate the design. If the problem occurs during production quality control then the switch may be discarded as the production line may be checked.